The invention relates to clock control systems and more particularly to a method and apparatus for reducing interference caused by electromagnetic radiation.
In present-day data processing systems, clock control processors are utilized in which clock generated timing signals are used for synchronizing the operation of the processor. The clock generator generally comprises some form of oscillator for generating clock signals of a predetermined frequency. Electromagnetic radiation such as radio frequency emissions are generated from the constant frequency clock signals and their harmonics as well as repetitive signals and their harmonics. Because the emissions are the result of constant frequency clock signals, spikes of radio frequency emissions are generated at regular intervals throughout the frequency spectrum as well as other spikes of less predictable frequency and reduced amplitude.
To reduce the electromagnetic radiation level produced by the clock timing signals, prior methods have included the use of shielding the system equipment, and the use of conductive enclosures, conductive gasketing and shield cables. Such techniques work on containing the signals within the system cabinetry rather than reducing the amplitude of the radio frequency emission signals directly. Where methods have been employed to reduce the signal amplitude, they have been directed to frequency modulating of the clock signal. Examples of this type of signal reduction can be found in U.S. Pat. Nos. 2,352,254 and 3,388,349 in which the system clock signals are modulated by a low frequency signal. Because the signals within most computing systems are defined to be at TTL levels, there is a limit as to how much reduction in the signal swing can be tolerated before the system ceases to work properly.
Accordingly, it is an object of this invention to provide a new and improved method for reducing interference caused by electromagnetic radiation from clock control systems.
It is a further object of this invention to reduce interference caused by electromagnetic radiation without shielding the system equipment. It is another object of this invention to reduce electromagnetic radiation by a method which is simple and economical and which overcomes the disadvantages of prior known methods.